Include WHERE in pattern comprehension tests

[szarnyasg]

The current acceptance tests for pattern comprehensions do not test for WHERE. An example is shown in the official Cypher docs. The WHERE clause is also allowed by the openCypher grammar:

openCypher/grammar/basic-grammar.xml

Lines 396 to 403 in c1a86ad

	<production name="PatternComprehension">
	[ &WS;
	<opt><non-terminal ref="Variable"/> &WS; = &WS;</opt>
	<non-terminal ref="RelationshipsPattern"/> &WS;
	<opt>WHERE &WS; <non-terminal ref="Expression"/> &WS;</opt>
	| &WS; <non-terminal ref="Expression"/> &WS;
	]
	</production>

I stumbled upon this while working on slizaa/slizaa-opencypher-xtext#17.